Wozniacki, the 2011 Family Circle Cup champion and this year's No.2 seed, won first, losing just nine points in seven service games and needing exactly an hour to beat Sílvia Soler-Espinosa, 61 61.

"I felt like I served really well today, and returned really well too," Wozniacki said after the match. "Playing your first match of the year on clay, it's always going to be difficult to slide and move unbelievably, but I felt like I did a pretty good job of that out there and I'm pleased with my work."

Awaiting Wozniacki in the next round is Andrea Petkovic, who came back from a break down in both sets - and saved a set point in the first set - to move past Vania King in straight sets, 75 64.

"Petkovic is a good player," Wozniacki said. "She has been very unlucky in her injuries and she is obviously a player that plays on a very high level, getting to the Top 10 before. She's dangerous."

Stosur, the 2010 Family Circle Cup champion and this year's No.3 seed, followed Wozniacki onto Billie Jean King court and beat Marina Erakovic in an hour and 13 minutes, 61 62. Stosur was almost as lethal as Wozniacki on serve, holding all eight of her service games during the straight-setter.

"I'm really pleased with the way I went out there and played today," Stosur said after the match. "I went for my serve a lot and whenever I had the opportunity I tried to get the ball off the court. That's why it really worked so well and helped me think out there, 'Okay, this is going well, I've got to keep at it.'"

Stosur was playing her first match since withdrawing from her quarterfinal match at Indian Wells with a calf injury. "I probably didn't expect to win my first match back one and two," Stosur commented. "But as the match went on I felt really good and the way I was playing all came together. And of course I really enjoy playing on this court, so I always try to do my best every time I'm out there."

She talked about not being in the clear just yet, though: "I mean it's all great but I don't want to get ahead of myself. I certainly want to take it day by day, keep doing the right things as far as getting the physio done and the treatment, and then wake up tomorrow and hopefully it will all be fine."

Next up for Stosur is Eugenie Bouchard, who reeled off five straight games from 1-all in the third set to upset No.16 seed Laura Robson, 64 36 61. Stosur and Bouchard will be a first-time meeting.

Robson wasn't the only seed to fall in the day session, with Mallory Burdette coming back from a break down in both the second and third sets to take out No.15 seed Sabine Lisicki, 36 76(5) 75.

No.5 seed Venus Williams became the last player into the round of 16, dropping a tough 58-minute second set but rebounding in the third to edge Mónica Puig in the feature night match, 62 57 63.

"She really found her rhythm in the second set," Williams said afterwards. "She started controlling the points, so in the third set I just tried to play confident and hopefully get some first serves in. But she played really, really well - she should be looking forward to a lot of great wins in the future."